Add fsfreeze_thaw_cycle()

As SIGTERM signals are intercepted by SignalTerminationGuard,
we just call `FIFREEZE` and `FITHAW` one after the other
without doing a complicated dance like in ostree.

Author: champtar

src/freezethaw.rs

@@ -0,0 +1,50 @@
+use rustix::fd::AsFd;
+use rustix::ffi as c;
+use rustix::io::Errno;
+use rustix::ioctl::opcode;
+use rustix::{io, ioctl};
+
+use crate::util::SignalTerminationGuard;
+
+fn ioctl_fifreeze<Fd: AsFd>(fd: Fd) -> io::Result<()> {
+    // SAFETY: `FIFREEZE` is a no-argument opcode.
+    // `FIFREEZE` is defined as `_IOWR('X', 119, int)`.
+    unsafe {
+        let ctl = ioctl::NoArg::<{ opcode::read_write::<c::c_int>(b'X', 119) }>::new();
+        ioctl::ioctl(fd, ctl)
+    }
+}
+
+fn ioctl_fithaw<Fd: AsFd>(fd: Fd) -> io::Result<()> {
+    // SAFETY: `FITHAW` is a no-argument opcode.
+    // `FITHAW` is defined as `_IOWR('X', 120, int)`.
+    unsafe {
+        let ctl = ioctl::NoArg::<{ opcode::read_write::<c::c_int>(b'X', 120) }>::new();
+        ioctl::ioctl(fd, ctl)
+    }
+}
+
+/// syncfs() doesn't flush the journal on XFS,
+/// and since GRUB2 can't read the XFS journal, the system
+/// could fail to boot.
+///
+/// http://marc.info/?l=linux-fsdevel&m=149520244919284&w=2
+/// https://github.com/ostreedev/ostree/pull/1049
+///
+/// This function always call syncfs() first, then calls
+/// `ioctl(FIFREEZE)` and `ioctl(FITHAW)`, ignoring `EOPNOTSUPP` and `EPERM`
+pub(crate) fn fsfreeze_thaw_cycle<Fd: AsFd>(fd: Fd) -> anyhow::Result<()> {
+    rustix::fs::syncfs(&fd)?;
+
+    let _guard = SignalTerminationGuard::new()?;
+
+    let freeze = ioctl_fifreeze(&fd);
+    match freeze {
+        // Ignore permissions errors (tests)
+        Err(Errno::PERM) => Ok(()),
+        // Ignore unsupported FS
+        Err(Errno::NOTSUP) => Ok(()),
+        Ok(()) => Ok(ioctl_fithaw(fd)?),
+        _ => Ok(freeze?),
+    }
+}

src/main.rs

@@ -33,6 +33,7 @@ mod efi;
 mod failpoints;
 mod filesystem;
 mod filetree;
+mod freezethaw;
 #[cfg(any(
     target_arch = "x86_64",
     target_arch = "aarch64",